IV-1

Round-Trip Missions to Low-Delta-V Asteroids and Implications
for Material Retrieval

DAVID F. BENDER, R. SCOTT DUNBAR and DAVID J. ROSS

Low-delta- V asteroids are to be found among those which have
perihelia near 1 AU. From the 50 known asteroids with perihelia
less than 1.5 AU, 10 candidates for asteroid retrieval missions
were selected on the basis of low eccentricity and inclination. To
estimate the ranges of orbital elements for which capture in Earth
orbit may be feasible, a survey was made of 180o transfers from a set of orbits having elements
near those of the Earth to the Earth. For 2 of the 10 low-delta-V
asteroids and for an additional one with elements more Earth-like
than any yet known, direct ballistic round trips in the 1980's were
computed. A stay time of several months at the asteroid was used.
The results show that the total delta V, including that for
rendezvous with Earth upon return, for the known asteroids is above
14 km/sec. But if asteroids are found similar to the strawman
considered, the total delta V could be as low as 10 km/sec.

To obtain trajectories with lower delta V than the direct
ballistic cases, three modifications to the trajectories that use
gravity-assisted flybys are considered: a lunar gravity assist at
both departure and arrival at Earth, Earth-Venus-Earth flights
which can greatly increase or decrease heliocentric energy, and
low-thrust Earth-to-Earth transfers to increase or decrease the
V at Earth.

Ballistic studies using Earth-Venus-Earth return trajectories
were made for 3 of the 10 low-delta-V asteroids. These studies can
form the basis for low-thrust return trajectories, but only one
low-thrust-case was investigated. Low-thrust mission results are
shown for a direct retrieval of the "Earth-like " asteroid and a
gravity-assisted retrieval of 1977 HB Although they have the
disadvantage of longer mission times, the gravity-assisted
trajectories make available for possible return a much wider range
of targets, as opposed to the severely restricted class of orbital
elements required for direct missions; for example, the 1977 HB
return requires a delta V of only 3.04 km/sec using low thrust and
gravity assists.

The final phase of the return of an asteroid is capture by
the Earth-Moon system, which is accomplished by a lunar gravity
assist even though V at
Earth arrival is as high as 1.5 km/sec. A sample trajectory showing
capture into an orbit near the 2:1 Earth-Moon resonance is
presented.

With the discovery of new Earth-approaching asteroids for
direct missions and more detailed and exhaustive trajectory studies
for gravity-assisted missions, it is felt that asteroid round trips
with delta-V values as low as 6 km/sec can be found. This work
serves only to demonstrate the feasibility of asteroid return
missions and to indicate possible directions for future
studies.

INTRODUCTION

Many types of asteroid missions, including flybys (refs. 1, 2), rendezvous (ref. 3), and sample return missions (ref. 4) have been discussed previously.
Recently, the idea of retrieving all or part of an
Earth-approaching asteroid as raw material for space manufacturing
has been proposed. This paper addresses the problems of trajectory
dynamics crucial to this scheme and proposes several techniques for
conducting such a mission.

Clearly, only those asteroids accessible with the lowest
possible delta V to rendezvous will be of interest, at least for
the time being. Such low-delta-V asteroids will be those crossing
or inside the orbit of Mars, with small eccentricity, low
inclination, and a semimajor axis near 1 AU.

In addition to trajectories that travel directly from Earth to
an asteroid and return, the greatest possible use will be made of
gravity assists. Earth escape and capture will be facilitated by
lunar-gravity assists. In the solar system, flybys of Earth and
Venus will be used to shape the trajectory, with a consequent
reduction in total delta V. Finally, the possibility of low-thrust
trajectories between gravitational encounters will be considered
briefly. (However, it was not possible to explore this concept in
depth here.)

LOW-DELTA-V ASTEROIDS

A low-delta-V asteroid is one for which the total impulse to
rendezvous, including Earth launch delta V, is among the lowest
known. In this study, the return of a significant part of the
asteroid is considered and thus the total impulse to return to
Earth, including that for rendezvous with Earth, must also be as
low as possible. Generally, the outbound and return thrust
requirements should be essentially the same, but to return an
asteroid, contrary to expectation, the outbound delta V must be
minimized at the expense of the inbound delta V (ref. 5).

The low-delta-V asteroids will be found among those which come
near the Earth; such a list is obtained by arranging all the
asteroids by perihelion distance (Ql). Orbital elements of the 53
asteroids with perihelion under 1.5 AU are given in table 1.

The designation (L) after the name indicates that the asteroid
is lost. The elements listed are semimajor axis (A), eccentricity
(ECC), inclination to the ecliptic (INCL) and aphelion distance
(Q2). Two of the 53 asteroids - 1977 HA and 1977 HB - were
discovered in April 1977. In addition, orbital elements for seven
large fireballs (meteoroids) are added to indicate a range of
orbital elements for large objects that have intersected the
Earth's orbit.

In selecting the low-delta-V asteroids from this list, note that
a delta V of 6 km/sec for Earth escape to asteroid rendezvous or
for return would allow the spacecraft to reach to 1.8 AU for e = 0.45 and
i = 0, or to i = 12o for a = 1 AU and e = 0. These
values, namely a
1.8 AU, e 0.45,
and i
12o, are used as limits in
selecting asteroids although by doing so regions are included for
which the V would exceed
6 km/sec. Six asteroids are thus selected from table 1: 1976 UA, 1977 HB, Toro, 1959 LM,
1973 EC, and Eros. Another seven lie near but outside this
boundary: Adonis, Hermes, Apollo, 6743 PL, Geographos, Amor, and
Ivar. If the three lost asteroids are excluded from this set of 13,
10 may be considered as low-delta-V asteroids (see table 2).

To estimate in a general way the delta-V values for return of
material from asteroid orbits, we first examined the ballistic
equivalent trajectories for a general class of objects with orbital
parameters near those of Earth. The range of semimajor axes was
from 0.8 to 1.2; of eccentricities, from 0 to 0.4; and of
inclination, from 0o to
10o. These limits approximate
those developed above by setting a bound on the delta V at Earth of
6 km/sec to produce an elliptical orbit or an inclined orbit. This
value is higher than can reasonably be provided by available thrust
systems operating on an asteroidal mass, but these limits encompass
the ranges of orbital elements that will be of interest.

The delta-V values are computed for 180o ballistic transfer from the asteroid orbit to
Earth. For no inclination of the asteroid orbit, the total delta V
is known to be optimal. Figure 1
shows delta V at the asteroid as a function of eccentricity and
semimajor axis of the asteroid orbit. The contours of constant
delta V are marked with the delta V (in kilometers per second) and
a symbol to indicate whether the departure is from the aphelion of
the asteroid orbit (number within triangle) or from the perihelion
(encircled numbers). The delta V at Earth to yield rendezvous is
indicated only for 1.5 km/sec, which is taken to be the upper limit
of the approach velocity for "free" capture with a lunar gravity
assist (dashed lines).

For transfers from inclined orbits to Earth, the first impulse
is applied at a node and is assumed to establish a transfer orbit
that is coplanar with and tangent to the Earth's orbit
180o from the node. For
simplicity, only orbits with the nodes at perihelion and aphelion
are considered. By departing from these nodes (which can be
characterized as from perihelion or from aphelion), the same kind
of curves can be presented for orbits of any single inclination as
in figure 1 for coplanar orbits.
Figure 2 and figure 3 show delta V at the asteroid as
a function of a and e for inclinations of
2o and 5o, respectively. The dashed lines in figure 1 , showing the free capture
region, transfer identically to figure 2 and figure 3 since the transfer orbits are
the same for each figure.

For any given semimajor axis and eccentricity, one of the two
transfers in figure 2 and
figure 3 will have less total
delta V than the other. It can be shown that for various locations
of the periapse around the orbit the optimum transfer will be
between these two values.

Of the 10 low-delta-V asteroids from table 2, only 1976 UA lies near the
regions plotted in figures 1-3. Its position is indicated in
figure 3, but, because it has an
inclination of 5.9o, the delta-V
values from the graph are too small. In an effort to present an
example of a moderate delta-V requirement, a fictitious asteroid,
Ames SSS 77, is hypothesized with the following elements: semimajor
axis, 1.08 AU; eccentricity, 0.18; inclination, 5o; longitude of ascending node, 0o; argument of perihelion, 45o; and mean anomaly, 0o, on 14 November 1979.

The position of this asteroid is also marked in figure 3, which indicates that the
delta-V value to depart from the asteroid to Earth can be as low as
2.3 to 3.4 km/sec (if the relative positions of Earth and the
asteroid are those required for the transfer). The capture by Earth
should ideally require only a small delta V to reduce the approach
speed to the 1.5 km/sec assumed as satisfactory for lunar-assisted
capture.

Suppose that delta V at the asteroid is limited to 3 km/sec and
the Earth approach speed is limited to the "free" capture speed of
1.5 km/sec. Then, from figure 1,
a zigzag fine across the figure as shown by the arrows shows which
regions of a and e are accessible. In figure 2, the corresponding fine is
slightly modified from that in figure 1, but for 5o inclination (fig. 3), the available ranges for
a and e are reduced significantly.

POSSIBILITIES FOR GRAVITY-ASSISTED ASTEROID MISSIONS

Consider a spacecraft moving in the gravitational field of a
central body and assume that a moderately massive secondary body is
also moving in the field. The spacecraft can be directed to
approach the secondary body closely, causing the spacecraft to
change its direction and magnitude of velocity in the field of the
central body. It is assumed to occur essentially instantaneously at
the position of the secondary body. A gravity-assisted flyby uses
this change in spacecraft velocity to enhance the mission under
consideration.

For asteroid sample return missions, and particularly for
asteroid retrieval, a gravity assist by the Moon in the Earth's
gravity field would lower the delta-V requirement significantly. It
has been shown that capture by the Earth-Moon system can occur for
a hyperbolic approach velocity to the Earth as high as 1.85 km/sec
(ref. 6). Similarly, an escape velocity
of 1.85 km/sec can be achieved on a parabolic trajectory in the
Earth's field by a lunar flyby on the outbound leg. These cases
require a grazing flyby of the Moon. To provide a safe margin for
guidance, V = 1.5 km/sec is used at Earth approach or departure.
Multiple flybys of the Earth-Moon system with a lunar gravity
assist at each could be used to capture at a higher velocity
(ref. 6), but such trajectories
considerably lengthen total mission time (1 yr per pass). For two
flybys, the upper limit is 2.58 km/sec. An example of a single
encounter capture is examined in a section (Lunar Gravity-Assisted
Capture).

Multiple flybys of a single secondary body cannot be used to
change the magnitude of the hyperbolic approach speed to that body
(if its orbit is circular), assuming that no delta V is applied
along the trajectory between encounters. But if the trajectory is
modified by a gravity assist at a different body or by impulsive or
low-thrust delta V between encounters, dramatic changes can be
made. Bender and Friedlander (ref. 2)
have shown that, by means of a Venus-Earth gravity-assisted
trajectory, almost any asteroid in the main belt can be approached
on a ballistic trajectory with lower than direct ballistic energy
expenditure. In that study launch velocities of 3 to 5 km/sec from
Earth were considered. The Venus flyby modifies the orbit so that
the subsequent Earth flyby is at a higher relative speed. The Earth
return speeds used to reach the asteroid belt and beyond were in
the range of 8 to 14 km/sec. Moreover, trajectories were found for
dates of the Earth return flyby over a large fraction of any
synodic period of Earth and Venus.

Presumably, if any asteroid can be reached by an
Earth-Venus-Earth trajectory, return trips are also possible with
arrival conditions at Earth similar to the low-energy launches
considered by Bender and Friedlander (ref.
2). Additionally, for an asteroid to be retrievable by means of
Earth and Venus gravity assists, the asteroid must approach Earth
or Venus so closely that only a small delta V is required for the
first encounter.

A technique little studied to date is to shape a trajectory by
low thrust between gravitational maneuvers. This concept is applied
here to reduce an Earth flyby speed from 3-5 km/sec at return from
Venus to 1.5 km/sec as needed for the final capture. Table 3 shows some test results obtained
when the flyby speed was reduced by low-thrust propulsion between
two Earth encounters. Equivalent delta V is the total delta V
supplied by the low-thrust system. Low-thrust propulsion
effectively reduces the relative speed at Earth arrival. Capture by
the EarthMoon system can be managed in 410 days or less at approach
speeds from 3 to 5 km/sec with a low-thrust delta V that provides
from 1 to 2.7 km/sec.

If this capture process is combined with Venus-Earth gravity
assists, a great range of asteroid orbits is opened for asteroid
retrieval. The low-thrust system is required mainly to provide for
the first Earth or Venus encounter and the capture phase. The only
requirement is that the total velocity change required to produce a
flyby of Earth (or Venus) be small. The known candidates are listed
in table 2. A Venus encounter
requires that the perihelion distance be less than 0.72 AU; a
crossing of the Venus orbit plane at Venus's distance from the Sun
is unlikely. An Earth encounter similarly requires that the
perihelion distance be less than 1.0 AU. But crossing the Earth
orbit plane near 1 AU is very likely since this is essentially the
condition required for discovering it. Results of testing a few
asteroids from table 1 indicate
that a delta V of about 500 m/sec represents the lower range of
values for achieving the first Earth encounter. The total delta V
for the return phase of an asteroid retrieval will be from 2-4
km/sec, possibly sometimes as low as 1.5 km/sec.

If the smaller fraction of the total delta V is needed at the
start of the retrieval process, it and the navigational delta-V
values required to control the flybys might be supplied by a
relatively modest mass driver. The final larger delta V, upon Earth
return from Venus, could be supplied by a larger mass driver sent
to rendezvous with the asteroid in its nearly final and much lower
delta-V orbit, provided sufficient time remains to accumulate the
required delta V.

BALLISTIC RESULTS

Direct Retrievals

Ballistic mission studies for asteroid sample return were
performed for several asteroids, including some of interest for
possible asteroid retrievals. Although there is little possibility
of ballistic asteroid retrieval, ballistic studies do show the
relative costs of reaching and bringing back various asteroids. The
delta-V values found will not differ greatly from the low-thrust
delta-V values.

Actual ballistic round trips to two asteroids 1977 HB and 1974
EC are shown in table 4.
Additional data are shown for the hypothetical asteroid Ames SSS 77
(table 5) in a later opportunity,
which is representative of those that might be discovered. The fact
that the real departures and arrivals with a considerable stay time
at the asteroid have been required means that delta V will be
considerably larger than for the ideal two impulse transfer between
the orbits.

Table 4 is divided into three
parts: dates and time intervals, impulses needed, and orbit
elements.

The impulse to remove V at Earth is included in the total delta V
because the asteroid must be captured. All legs were optimized on
total delta V for rendezvous. Ecliptic plane projections of the
orbits of SSS 77 and 1977 HB are illustrated in figures 4 and 5 drawn to the same scale so that
differences are easily observed. Ames SSS 77 is significantly more
accessible than 1977 HB. Note- that in table 4 the use of lunar gravity assist
on Earth departure and arrival is not included. If this "free"
delta V were 1.5 km/sec, the total delta V would be reduced by 3
km/sec. Note also that the total impulse required on the retrieval
phase for Ames SSS 77 is 3.76 + 1.68 - 1.5 or 3.94 km/se, which
lies between 3 to 4 km/sec, postulated previously for the minimum
total delta V to retrieve an asteroid by direct flight to
Earth.

Earth-Venus-Earth Gravity-Assisted
(EVEGA)Retrievals

As mentioned in the preceding section, one method for reducing
overall delta V during retrieval is to use gravity assists by Earth
and Venus. In this technique, the orbital inclination and the high
excess hyperbolic velocity at the first Earth encounter can be
decreased to values that are amenable to a relatively easy capture
maneuver into high Earth orbit. The cost is the flight time to
Venus and back to Earth in one to three revolutions. A basic
technique of searching for mission opportunities of this kind has
been established but, as expected, a high velocity at the first
Earth approach cannot be reduced to low values in two or three
revolutions for every date. In the data below, no more than three
revolutions were considered.

The scheme of using an EVEGA trajectory to retrieve an asteroid
is as follows: (1) the asteroid is driven from its orbit by some
sort of propulsion system (high or low thrust) so as to intercept
the Earth, (2) the encounter is controlled so that the spacecraft
flies on to Venus (ideally, with no intermediate thrusting during
the flyby), and (3) it then makes a gravity-assisted flyby of
Venus, sending it back to Earth for rendezvous. In searching for
mission Opportunities of this type, each separate leg was analyzed
ballistically in hopes of finding a suitable set of launch/arrival
dates so that the hyperbolic excess velocities, Vin and Vout at each boundary
point would be matched as closely as possible and the velocity at
the second Earth approach would be minimized (i.e., from 3-5
km/sec). Given the rough boundary dates derived from this survey,
the entire case could be run at once and optimized on total delta V
from asteroid to Earth to Venus to Earth. The results of four such
cases are given in table 6.

These results indicate that the Earth-Venus-Earth gravity-assist
technique will contribute significantly to the retrieval of
asteroid material. The flyby velocity at the first Earth encounter
on the return trip is sometimes too fast and sometimes too slow to
match the "best" value for the gravity-assist geometry that occurs.
It is therefore conceivable that, when more asteroids are found, it
will be possible to obtain cases for which there is a closer match
of Vin and
Vout at
Earth. The earth return velocities are sometimes too high, but it
is believed that, if only cases for which this velocity is under 5
km/sec are considered, there will still be many opportunities.
These are ballistic results and the addition of low-thrust
techniques to those of gravity assist will result in successful
asteroid retrieval trajectories in almost every case. One such case
developed from the 1977 HB results is presented in table 7.

LOW-THRUST RESULTS

It has been shown that the total delta V for retrieving an
asteroid is likely to range from 2 to 4 km/sec. To estimate the
capability of a mass driver in supplying such a delta V, suppose
that delta V is supplied at the rate of 1 km/sec/yr to a
200-m-diameter asteroid with a mass of 1010 kg (density = 2.39 g/cm3). The acceleration is 31.7XI0-6 m/sec2 and,
if the mass is expelled at 5000 m/sec, the power required is 792 MW
and the mass flow rate is 63.4 kg/sec. These are strong
requirements on the mass-driver system and it may not be possible
to meet them. If not, the size of the asteroid would have to be
reduced or the flight time increased so that the total delta V
would be supplied at a slower initial rate.

Low-thrust techniques can be used to improve the ballistic
retrieval trajectories in table
6. A low-thrust trajectory can be used from the asteroid to
first Earth encounter in such a way as to supply the proper
magnitude of V at Earth.
For 1977 HB, this technique was combined with an additional
revolution about the Sun, that is, by starting one revolution
earlier from the asteroid orbit. The Earth flyby encounter and
velocity were obtained at a total delta V of 2.04 km/sec supplied
in 653 days. This slightly exceeds the acceleration value of 1
km/sec/yr adopted as a nominal upper limit. With a low-thrust
Earth-to-Earth trajectory at a delta V of 1.00 km/sec to reduce
V at capture from 3.04
to 1.5 km/sec (as in table 3),
the return of 1977 HB is possible with a total delta V of 3.04
km/sec. As shown in table 7., the
return begins from the asteroid orbit on 1 July 1985, with a
653-day low-thrust transfer to Earth; "free" flybys of Earth,
Venus, and Earth in succession; and, finally, a 390-day low-thrust
trajectory before final Earth encounter on 23 September 1989.

Only navigational delta-V values are needed during flyby.
However, the low thrust could possibly be applied during the
Earth-Venus-Earth portion to further reduce the total delta-V
requirement for capture. (No analysis of this variation in the
technique was attempted here.)

Finally, searches were made for suitable gravity-assisted
opportunities for an outbound trajectory to 1977 HB. Unfortunately,
no satisfactory gravity-assisted trajectories could be found using
less than three revolutions about the Sun with significantly less
delta V than that of the optimum direct impulsive case.
Consequently, the retrieval scenario in table 7 for 1977 HB contains the
impulsive data for the Earth-to-asteroid portion. The total delta V
given in table 7 reflects the
assumption that V = 1.5
km/sec represents the launch or capture assistance provided by a
lunar gravity assist.

It is realized that the total outbound delta V to asteroid
rendezvous is more important than the return delta V in terms of
providing the maximum return per unit cost (ref. 5). Unfortunately, the 1977 HB case was
developed naively on the basis of minimizing the return delta V.
The present outbound results remove it from serious consideration.
None of the other cases in table
6 were investigated, but the outbound results for 1977 HB were
instructive and indicate that satisfactory cases should be found -
especially when more Earth-approaching asteroids are known.

LUNAR-GRAVITY-ASSISTED CAPTURE

As mentioned previously, it is reasonably safe to use a
lunar-gravity-assisted maneuver to capture an Earth-approaching
body with a V relative
to the Earth of less than about 1.5 km/sec. At 1.85 km/sec, the
capture orbit is almost parabolic and there is no freedom in
choosing the radius of perigee. In considering the capture problem
ballistically, it is desirable that the radius of perigee of the
capture orbit equal the radius of perigee of the final desired
orbit so that injection into the desired orbit can be accomplished
in one maneuver. Because of its stability and favorable
gravitational characteristics, the orbit in 2:1 resonance with the
Moon, described by Heppenheimer (ref.
7), was chosen for this example (fig. 6). For a sufficient margin of
safety and one that ensures capture of the approaching body into
Earth orbit, a V =
X lunar orbital
velocity (= 1.447 km/sec) is considered. The body passes 184 km
above the lunar surface, which is considered an adequate margin of
safety, and has its orbit relative to the Moon bent
89.5o and its Earth-relative
velocity reduced from 2.046 to 1.316 km/sec. It is injected into an
orbit having a radius of perigee of 0.3952 X lunar distance.

After the lunar-gravity-assisted flyby, an impulsive velocity
change of 292.3 m/sec at the perigee is sufficient for injection
into the 2:1 orbit. In practice, since the stability of the 2:1
orbit depends on the avoidance of close lunar approaches by having
the body at apogee when the Moon is 90o away, injection into the 2:1 orbit would be a
two-stage process. First, a near 2:1 orbit would be achieved and
allowed to coast until a suitable relationship with the Moon had
been established and a second velocity change performed to lock the
orbit into resonance. For a low-thrust mission, the capture orbit
would be gradually changed over several periods to accomplish the
same thing with a considerably higher equivalent delta V.

This simple scenario ignores two interesting possibilities. The
first is the use of continuous thrust after lunar encounter to
further retard the body. A body with incoming V = 2.25 km/sec injected into a hyperbola
with perigee at 6 Earth radii may be captured with a continuous
thrust under 5 microgee (5XI0-5
m/sec 2) applied during the time
the body is within the lunar orbit. Thus bodies with approach
velocities considerably higher than the nominal 1.5 km/sec could be
captured with minimal effort.

The second interesting consideration ignored by this analysis
concerns multiple lunar encounters with a low-thrust interplanetary
trajectory between the Earth approaches. Preliminary investigation
indicates this may be very promising, but more thorough analysis is
not possible at this time.

CONCLUSIONS

It has been shown that direct round-trip trajectories for
asteroid retrieval missions may be possible with a total delta V of
the order of 7 km/sec, assuming that lunar-gravity-assisted flybys
are used both inbound and outbound. This would require a
significant increase in the number of known Earth-approaching
asteroids such that several with more favorable orbital elements
for this type of mission are found. The total delta V for the real
asteroid 1943 is about 11 km/sec.

On the other hand, gravity-assisted trajectories to rendezvous
with an asteroid and to return it to Earth may have total delta-V
values as low as 6 km/sec. This requires fortuitous timing and
geometry so that good Earth-Venus-Earth trajectories are available
for both outbound and inbound flights. However, they do not require
as limited a class of orbital elements for suitable targets as are
needed for direct retrieval. In fact, several candidates are known,
and every asteroid discovered by virtue of a close approach to
Earth becomes yet another candidate. A return trajectory using low
thrust for the asteroid 1977 HB was found with a delta-V
requirement of only 3.04 km/sec. Although no satisfactory outbound
flight arriving at this asteroid less than 1 year before the return
flight was found, it is confidently expected that, as asteroids are
studied and more are discovered, cases will be found that do indeed
provide retrieval for a total (round-trip) delta V of 5 km/sec.

Another technique involves the use of Earth-to-Earth low-thrust
trajectories. These are used to increase the V of a spacecraft at launch from 1.5 km/sec
to 3-5 km/sec (or to decrease V. for Earth approach from 3-5 km/sec
to 1.5 km/sec).

Finally, the problem of capture into long-term stable orbits
about the Earth was examined. A satisfactory procedure for entering
resonant orbits was developed and data presented for entering the
2:1 resonant orbits for V = 1.5 km/sec at Earth approach. It appears from
these studies that a significant amount of asteroidal material can
be retrieved by techniques well within the grasp of present
technology. More work is needed on the discovery of asteroids, the
search for good mission opportunities, and possibly the development
of other useful techniques to return an asteroid which will be
technically and economically feasible. In addition, precursor
missions to establish asteroid composition and structure are
essential.